The compound titanium nitride (TiN) has numerous potential applications because it is extremely hard, chemically inert (although it readily dissolves in hydrofluoric acid), an excellent conductor, possesses optical characteristics similar to those of gold, and has a melting point around 3000.degree. C. This durable material has long been used to gild inexpensive jewelry and other art objects. However, during the last ten to twelve years, important uses have been found for TiN in the field of integrated circuit manufacturing. Not only is TiN unaffected by integrated circuit processing temperatures and most reagents, it also functions as an excellent barrier against diffusion of dopants between semiconductor layers. In addition, TiN also makes excellent ohmic contact with other conductive layers.
In a common application for integrated circuit manufacture, a contact opening is etched through an insulative layer down to a diffusion region to which electrical contact is to be made. Titanium metal is then sputtered over the wafer so that the exposed surface of the diffusion region is coated. The titanium metal is eventually converted to titanium silicide, thus providing an excellent conductive interface at the surface of the diffusion region. A titanium nitride barrier layer is then deposited, coating the walls and floor of the contact opening. Chemical vapor deposition of tungsten or polysilicon follows. In the case of tungsten, the titanium nitride layer provides greatly improved adhesion between the walls of the opening and the tungsten metal. In the case of the polysilicon, the titanium nitride layer acts as a barrier against dopant diffusion from the polysilicon layer into the diffusion region.
At least five techniques are currently available for creating thin titanium nitride films having low bulk resistivity: reactive sputtering; annealing of an already deposited titanium layer in a nitrogen ambient; a high-temperature atmospheric pressure chemical vapor deposition (APCVD) process, using titanium tetrachloride, nitrogen and hydrogen as reactants; a low-temperature APCVD process, using ammonia and Ti(NR.sub.2).sub.4 compounds as precursors; a LPCVD; and a low-pressure CVD process using ammonia and Ti(NMe.sub.2).sub.4 as precursors. Each of these four processes has its associated problems.
Both reactive sputtering and nitrogen ambient annealing of deposited titanium result in films having poor step coverage, which are not useable in submicron processes. Chemical vapor deposition processes have an important advantage in that a conformal layers of any thickness may be deposited. This is especially advantageous in ultra-large-scale-integration circuits, where minimum feature widths may be smaller than 0.5.mu.. Layers as thin as 101/8 may be readily produced using CVD. However, TiN coatings prepared used the high-temperature APCVD process must be prepared at temperatures between 900.degree.-1000.degree. C. The high temperatures involved in this process are incompatible with conventional integrated circuit manufacturing processes. Hence, depositions using the APCVD process are restricted to refractory substrates such as tungsten carbide. The low-temperature APCVD, on the other hand, though performed within a temperature range of 100.degree.-400.degree. C. that is compatible with conventional integrated circuit manufacturing processes, is problematic because the precursor compounds (ammonia and Ti(NR.sub.2).sub.4) react spontaneously in the gas phase. Consequently, special precursor delivery systems are required to keep the gases separated during delivery to the reaction chamber. In spite of special delivery systems, the highly spontaneous reaction makes full wafer coverage difficult to achieve. Even when achieved, the deposited films tend to lack uniform conformality, are generally characterized by poor step coverage, and tend to deposit on every surface within the reaction chamber, leading to particle problems. Finally, the low-pressure CVD process, though producing layers of uniform thickness, does not provide acceptable step coverage for high aspect ratio trenches.